1. Field of the Invention
The present invention relates generally to the manufacture of nuclear fuel rods to be incorporated into fuel assemblies for nuclear reactors and, more particularly, is concerned with an improved apparatus for applying an end plug to a fuel rod tube end such that the end plug is guided in a secured manner into the tube end regardless of where the end plug falls in its diametral tolerance range.
2. Description of the Prior Art
Fuel elements or rods for nuclear reactors commonly encase the fissile material in thin walled cladding or tubes which serve to support the nuclear fuel during the operation of the reactor. The nuclear fuel, which is usually in the form of cylindrical pellets of enriched uranium dioxide, must be isolated from the environment surrounding the tubes to prevent contact and chemical reactions between the fuel and other materials such as water in a pressurized water reactor. Thus, the nuclear fuel is ordinarily hermetically sealed in each thin walled tube by the use of a pair of opposite end closures or plugs.
It is critical that the end plugs themselves be impermeable and mechanically strong to contain the fissile products. It is equally critical that the mechanical connection of each end plug with an end of each tube be free of defects such as discontinuities, cracks and tube distortions which could eventually produce leaks.
Present methods of applying the end plugs use close fitting guides to align the plugs with the tube, such as illustrated in FIG. 2. Due to manufacturing tolerances, the necessary clearance between the plug and guide sometimes permits the plug to cock, or tip, as shown in solid line form in FIG. 2, which causes a shaving action on the plug exterior as the plug is rammed home. Thereafter, when the connection is completed such as by welding the plug to the tube, faulty welds commonly result which produce leaking because of discontinuities in the weld joint or connection caused by this shaving action. Also, frequently the plug does not seat properly which increases the likelihood of leakage. Additionally, pieces of the plug are broken off occasionally as it is forced into the tube. The broken pieces not only damage the end plug but also cause foreign objects to be deposited in the fuel rod.
Consequently, a need exists for an improved technique for applying an end plug to the tubular end of a fuel rod which will accommodate slight variations in the dimensions of interfitting parts due to unavoidable manufacturing tolerances and thereby increase the resistance of fuel rods to leakage and failure.